This invention relates to dispersion of a nano-alumina; and, more particularly, to an improved nano-alumina dispersion for coatings such as wire coatings. Nano-alumina dispersions are used in many coating applications. In electrical insulation applications, nano-alumina dispersions that are thixotropic have been found to produce an even edge build-up on a shaped wire. It has also been found that low loading levels in a polyamideimide overcoat for the wire lowers the coefficient of friction, and improves abrasion resistance of the wire coating. It has further been found that high loading levels (−20% on resin solids) in polyester, polyesterimide, polyamideimide, polyimide or polyurethane coatings achieve a very acceptable corona resistance in inverter duty motors.
Alumina is typically available in a powder form. However, dispersion of the powder in a resin system, or solvent, presents problems. This is because the alumina forms insoluble aggregates that require extreme sheer forces to break down into individual particles. Typical ways of accomplishing this include ultrasound, ball milling, sand milling, and high pressure homogenization, for example. A problem with these and similar techniques, however, is that the resulting dispersion is often inconsistent with the result that the alumina particles settle or re-agglomerate in the resin or solvent system. This leads to coating non-uniformities and quality problems for the end user.
Dispersants commonly used in the coating industry can be used to mitigate these dispersion and settling problems. But, the high loading levels needed with nanoparticles, because of their large surface area, affect the usefulness of the dispersants. Also, the dispersants used are often found to be detrimental to the physical properties required in final, cured coatings. These include poor thermal stability and coating defects. The result is that the high costs incurred in using these dispersant cannot be easily justified.
Fillers such as alumina are common in the electrical insulation coating industry, and there are a number of U.S. patents directed at the use of fillers for improved corona endurance magnet wire. These patents include U.S. Pat. Nos. 6,649,661 and 6,476,083, for example. However, the use of fillers does not solve the problem either.